Microfluidics and lab-on-a-chip technologies, also referred to as microdevice technologies, have been proposed for use in the field of analytical and bioanalytical chemistry, particularly in applications that employ fluids that are rare, expensive, and/or available in limited amounts. Such applications include, for example, proteomics, and genomics. The small size of microdevices allows for the analysis of minute quantities of sample. In addition, because microdevices typically have a simple construction, they are in theory inexpensive to manufacture. Furthermore, the small size associated with microdevice may also contribute to increased detection/analytical sensitivity. Such increased sensitivity have been observed in certain nanoflow applications involving liquid chromatography and mass spectrometry.
Microdevices may also be advantageously used to integrate a plurality of functions into a single device. For example, a single microdevice may be adapted carry out a number of different separation techniques, including chromatographic separation techniques that are often preferred for samples containing analyte molecules with low electrophoretic differences, e.g., small drug molecules. Chromatographic separation occurs when a mobile phase carries sample molecules through a chromatography bed (stationary phase) where they interact with the stationary phase surface. The velocity at which a particular sample component travels through a chromatography bed depends on the component's partition between mobile phase and stationary phase.
Ordinary liquid chromatography techniques may be carried out by using a packed column, e.g., a column containing chromatographic separation beads of 1 to 20 μm in diameter. Mechanical or other types of pumps may be employed to generate sufficient pressures to drive a sample through the column. Such pressure-driven, bead-based separation technologies may be employed in conjunction with microfluidic approaches. For example, U.S. Patent Application Publication No. 20030017609 to Yin et al. describes a microdevice that employs pressure-driven flow to effect component separation in a fluid using chromatographic beads. Such pressure driven technologies may be used in conjunction with flow switching structures described in U.S. Patent Application Publication No. 20030015682 to Killeen et al.
Given the dimensional limitations and the surface force effects associated with microdevices, it is not a trivial matter to introduce chromatographic media into a microdevice conduit. Only a few techniques for packing and trapping such media in microdevice conduits are alluded to in the art. In general, these techniques require the use of microdevices having a relatively complex design and/or construction. In turn, specialized equipment may be necessary for manipulating the microdevices and/or loading of the chromatographic media into the microdevices. Furthermore, these techniques generally do not give reproducible results and are not easily adaptable for automation.
For example, International Publication No. WO 01/38865 describes a packing method that employs electrokinetic flow to transfer particles into a chamber within a microfluidic device. The chamber is associated with weir structures that serve to confine the particles therein. However, this method is difficult to implement for at least two reasons. First, electrokinetic flow is practicable only with certain combination of channel surfaces and fluids. Second, weir structures of microfluidic devices require exacting dimensional precision for operability. Such precision is generally difficult to achieve and maintain for microdevices made from flexible polymeric materials such as polyimide.
Weir structures are also described in U.S. Pat. No. 6,581,441 to Paul. Thus, the microdevice loading technology therein suffers from some of the same drawbacks as the technology described above. In addition, the microdevice loading technique described in this patent requires insertion of capillaries into the inlets of the microdevice to be loaded. As discussed in International Publication No. WO 01/85341, the capillaries may be subsequently glued on for permanent attachment of the capillary to the microdevice. Regardless of whether the capillary is permanently attached to the microdevice, the precision needed for such capillary insertion operations are not easily adapted for automation.
To avoid using frit structures, International Publication No. WO 02/075775 and related U.S. Patent Application Publication No. 20020142481 to Andersson et al. describe technologies that use centripedal forces to pack microconduits. To achieve the centripedal forces for microconduit loading, disc shaped microdevices are required. In addition, the microdevices are described as having a specialized microconduit construction to keep loaded particles in place. Thus, it should be evident that while this approach avoids the complexities associated the manufacture of weirs, such complexities are replaced with the manufacturing challenges associated with specialized microconduit designs and geometries required for the application of centripedal forces.
Thus, there is an unrecognized need in the art for improved apparatuses and methods for introducing particles into microdevices. The invention meets this need by employing standardized equipment with minor modifications of such equipment rather than specialized equipment to effect automatic handling and loading of microdevices in a reproducible manner. In addition, the invention is adaptable for use with microdevices of simple and complex constructions. Furthermore, the apparatuses and methods provided by the invention exploit novel ways to form particle bridges that may serve as a frit structure in a microconduit through the use of particles having a smaller diameter than the diameter of the microconduit.